The potassium content of human body fluids is regulated by virtue of epithelial transport processes in the colon and distal renal tubule which lead to the absorption or secretion of this ion. Understanding the mechanisms of potassium transport, and in particular the control of these processes by drugs and hormones, requires a detailed description of the properties of the ion pumps and ion channels in the apical and basolateral membranes of the transporting cells. Despite the obvious importance of epithelial potassium channels the properties of these transport elements have not been studied in detail because the series arrangement of apical and basolateral membranes precludes the direct measurement of isotope flows and electrical currents through the channels. We have developed a unique preparation of the isolated turtle colon in which the basolateral membrane is functionally isolated by using the polyene antibiotic, amphotericin-B, to modify the cation selectivity and conductance of the apical membrane. This preparation has permitted us for the first time to measure simultaneously the potassium current and tracer flows through the channel. Our initial studies revealed that the channel is permeable to at least three cations, potassium, challium and rubidium, and more importantly that these ions interact in a manner consistent with a single-file mechanism of ion flow. The ionic interactions are revealed in the form of positive coupling between the flows of permeant ions (knock effects) and reductions in ionic permeability of a test ion when a second ion is added (block effects). In addition we have shown that the divalent cation, barium, is a potent blocker of the channel as is quinidine, a blocker of the calcium-induced potassium conductance in human red blood cells. The object of the propoed research is to obtain, for the firt time, a detailed analysis of the mechanism of ion translocation through an epitelial potassium channel. We will determine the kinetics of single ion flows and determine the flux-ratios for K, T1 and Rb. We will study in detail the nature of cation-cation interactions which occur in the channel and investigate possible sideness or asymmetry between the cellular and extracellular aspects of the channel. We will attempt to determine if protons or water pass through the channel, and investigate the action of blociers; barium or quinidine.